1. Field of the Invention
The present invention relates generally to sensors for the measurement of force and load. The present invention relates more specifically to an electromagnetic sensing device for measuring changes in force or load through the measurement of stress-induced magnetic property changes. The present invention has specific application in measuring the changes in pressure in internal combustion engine cylinders and measuring seat occupant weight in passenger vehicles.
2. Description of the Related Art
Modern motor vehicles are becoming increasingly "intelligent" in their ability to detect and respond to operating characteristics in a manner that improves the safety and overall efficiency of the motor vehicle. For some time now, microprocessor-based controls have improved the efficiency of automotive engines by automatically adjusting such things as fuel mixture, timing, acceleration, speed, temperatures and a variety of other operating parameters. In addition, electronic controls and sensors have been utilized to handle such items as anti-lock breaking systems, emission controls, air bag deployment, climate controls, and engine maintenance warnings. These improved devices for monitoring and controlling the operation of an automobile, however, have brought with them the need to accurately and quickly detect changes in the operating characteristics of the vehicle. The detection of these characteristics is accomplished by the use of a variety of different sensors that report timing, temperature, pressure, load, wear, and other such quantitative values back to a microprocessor control system for use.
One of the most significant bits of information that can assist the efficient operation of a motor vehicle is an accurate measurement of the changes in the internal pressure for the cylinder combustion chambers in the engine. These changes are directly related to the combustion efficiency and, therefore, the overall efficiency of the motor vehicle. It is quite common to make pressure change measurements in a shop environment where the vehicle spark plugs can be removed and pressure characteristics within a cylinder can be measured. However, on board measurement of cylinder pressure is the more desirable approach for controlling the engine to achieve a higher combustion efficiency. It is only through this real time measurement that microprocessor based systems can adjust various engine functions to improve efficiency in light of various types and levels of pressure change. As an example, such information can be used to adjust ignition timing and the amount of fuel injected for optimum combustion as well as to monitor and control abnormal combustion events such as misfire and knocking.
Various methods and devices for on board sensing of cylinder pressures have been developed in the automotive industry. A number of such methods involve opening a whole in a spark plug or in the combustion chamber wall itself to provide a channel for the cylinder pressure to be transmitted to a detection location outside of the chamber or within the chamber wall. The actual gas pressure at the detection location is then measured by using a suitable pressure sensor. Other methods utilize annular piezoelectric pressure sensors that are clamped like washers between a spark plug and the spark plug seat in the chamber. Other applications of piezoelectric transducers include mounting the washer-like sensor to an engine head bolt where force changes can be measured and cylinder pressures can be inferred. Each of these latter methods, however, sense cylinder pressure changes indirectly since they rely on detecting clamping force changes caused by displacements of the spark plug or the engine head bolt that are due to cylinder pressures exerted on these engine components. This latter method of bolting a sensor to a critical engine component is preferred to the first method of opening a port because it is much less intrusive into the operating system of the engine. On the other hand, because it utilizes a piezoelectric element which is brittle and temperature sensitive, this latter sensing method has shown technical difficulties in achieving suitable durability and reliability. This is especially true for onboard operation where continuous temperature and vibration effects cause rapid deterioration and failure for such piezoelectric elements. The following patents disclose various sensor systems for detecting and measuring pressure changes in internal combustion engine cylinders: U.S. Pat. No. 2,879,450, issued in March 1959 to Baker; U.S. Pat. No. 5,101,659, issued to Takeuchi in April 1992, entitled "Mounting Device for Pressure Sensor"; U.S. Pat. No. 4,602,506, issued to Sawamoto et al. in July 1986, entitled "Combustion Pressure Sensor Arrangement"; U.S. Pat. No. 4,909,071, issued to Amano et al. in March 1990, entitled "Spark Plug Pressure Sensor"; U.S. Pat. No. 5,323,643, issued Kojima et al. in June 1994, entitled "Device for Detecting Change in Internal Pressure of Cylinder"; and U.S. Pat. No. 5,479,817, issued to Suzuki et al. in January 1996, entitled "Spark Plug With Build-In Pressure Sensor".
In terms of safety operations in a motor vehicle, many advances have been made in recent years towards improving the activation of safety devices when the vehicle is involved in a collision. The use of air bags in automotive vehicles to protect front seat occupants during a crash event has become standard in the industry. When activated, air bags are designed to inflate at a speed of up to 200 mph. Although this deploys the air bag in a time period sufficient to prevent the occupant from impacting components within the vehicle, this high inflation rate can impart a severe blow to the occupants in the process. This impact can be fatal if the occupant is an infant, child or small adult. There is a need, therefore, to distinguish between occupants in the front seat of a vehicle that are capable of handling the impact of an air bag and those that would be better served by other safety devices such as simple seatbelt systems. On concept being considered in the automotive industry to address the problem is the development of smart air bag systems that automatically deactivate the air bag when an infant, child or small adult is riding in the front seat. The key to implementing this approach is to accurately identify whether the front seat occupant requires air bag deactivation. The single factor most suitable for accomplishing this identification or classification of the occupant is the occupant's weight. There is, therefore, a need for a reliable and inexpensive weight sensor that can be used in association with air bag activation/deactivation systems. The following patents describe systems that have made attempts at identifying the presence and character of the occupant riding in the front seat of a vehicle and automatically deactivating the air bag as a result. These patents include: U.S. Pat. No. 5,413,378, issued to Steffens et al. in May 1995; U.S. Pat. No. 5,570,903, issued to Meister et al. in November 1996; and U.S. Pat. No. 5,573,269, issued to Gentry et al. in November 1996.
In each of the instances described above, there is a need for a sensor to measure force values associated with the efficient operation and the safe operation of the motor vehicle. In both cases, a sensor that can measure forces accurately and is durable over the long operational life of the motor vehicle would be desirable. Such a sensor would preferably be nonintrusive, i.e., would require very little modification to the existing engine and vehicle structure in order to implement, and rugged in that it should easily stand up to the temperature and vibration extremes associated not only with a vehicle engine, but with the structure for vehicle occupant support. The sensor device should also work easily in conjunction with existing monitoring and control systems currently being utilized for such vehicle safety and efficiency operations.